In a procedure for measuring the unbalance of a rotary member, for example a motor vehicle wheel, the rotary member is successively put into three operating conditions by a suitable drive arrangement, by means of an electric motor. In the first operating condition or phase, the rotary member which is subsequently to be balanced and on which therefore any unbalance has first to be measured is brought up to a speed of rotation which is referred to as the measuring speed of rotation. That phase of accelerating the rotary member to the measuring speed of rotation should take place as quickly and as smoothly as possible. In the second operating condition, when the rotary member is rotating at the measuring speed of rotation, any unbalance thereof is measured, with the rotary member being held at a speed which is as constant as possible. That phase should be as free as possible of oscillation and vibration phenomena which originate from the unbalance measuring assembly, more particularly the drive arrangement thereof. Finally, in the third operating condition or phase, after the unbalance of the rotary member has been suitably measured, the rotary member is slowed down and ultimately brought to a halt. That deceleration phase should also take place as quickly and as smoothly as possible.
Those requirements are best met by a drive comprising a three-phase motor which is connected to a three-phase network. If there is no three-phase network available, it is necessary to use a capacitor motor. That is a single-phase ac motor whose stator, in addition to the single-phase main winding in the main line of the power supply, also has an auxiliary winding which is electrically displaced through 90.degree.. An auxiliary current which is phase-shifted through 90.degree. can then be passed by way of the auxiliary winding. That phase shift is produced by a capacitor (operating capacitor) with which a starter capacitor is also connected in parallel, during start-up of the motor.
In operation of a single-phase ac motor of that kind, more especially a capacitor motor, it is found that pendulum oscillations, that is to say fluctuating angular speeds, of the rotor of the motor occur in the second operating condition of the unbalance measuring apparatus, that is to say during a measuring run during which any unbalance of a rotary member is measured while the rotary member is rotating at the at least substantially constant measuring speed of rotation. Such oscillations occur in particular in measuring runs involving a low level of load. For example, in the case of a balance machine for motor vehicle wheels, the mounting arrangement for carrying the rotary member is formed by a measuring spindle on which the wheel to be balanced is clamped. For the purposes of calibration or readjustment of the balancing machine, it may be necessary to carry out a measuring run without a wheel clamped on the measuring spindle, that is to say, a measuring run with a bare measuring spindle. That measuring run will tend to be impaired as a result of the above-mentioned pendulum oscillations or fluctuating angular speeds of the rotor of the motor, which will tend to cause the bare measuring spindle to rotate in an irregular fashion. That will not only adversely affect calibration of the machine but it can also give rise to a considerable amount of noise. The pendulum oscillations of the rotor of the motor, involving fluctuating angular speeds, are at double the frequency of the operating ac voltage (double the mains frequency) and, as has been found for the first time, are caused by an incomplete rotating field. In the usual capacitor motor, that rotating field is formed by a power supply circuit in which the operating capacitor is connected in series with the auxiliary winding and in parallel with the main winding.